(a) Field of the Invention:
The present invention relates to a shield machine having a plurality of rotating cutter heads so disposed in generally same planes that the distance between the centers of adjoining rotating cutter heads is larger than the radius, and smaller than the diameter, of the rotating cutter heads, and more particularly to a control system for the synchronous rotation of the cutter heads, wherein the cutter heads are so driven independently of each other by electric drive systems, respectively, each comprising plural motors with a reduction gear, as to rotate synchronously with each other and with such a difference in phase between the cutter heads as will not cause any interference between them during rotation.
(b) Description of the Prior Art:
As a shield machine suitable for use in building a railway tunnel, subway tunnel or vehicular tunnel having more than one lane, there has been proposed a multi-cutter shield machine having, a plurality of rotating cutter heads so disposed in generally the same plane such that the distance between the centers of adjoining rotating cutter heads is larger than the radius, and smaller than the diameter, of the rotating cutter heads, and which bore a tunnel of which the section takes the shape of a combination of two circles partially superposed, that is, a so-called cocoon or the like. Such shield machines are disclosed in, for example, Japanese Unexamined Patent Publications Sho 57-197395 and Sho 62-99597.
With such multi-cutter shield machines, in case, for example, two rotating cutter heads are disposed in substantially the same plane, the tunnel face areas of excavation by the respective rotating cutter heads can be made nearly equal to each other. Thus, it is easy to well balance the excavating resistances to the respective rotating cutter heads and so a one-side force which would otherwise adversely affect the posture of the entire shield machine will hardly occur. On the other hand, it is necessary to rotate the cutter heads synchronously with each other and with such a phase difference between the cutter heads that no interference will take place between them during rotation.
The typical conventional control systems or means for synchronous rotation of the adjoining cutter heads include the following:
(1) A control system in which a pinion fixed to the output shaft of the cutter drive motor is in mesh with a large gear fixed concentrically to each rotating cutter head and the large gears engage with each other to provide a mechanical synchronous rotation of the cutter heads.
(2) A control system in which the large gears are not in mesh with each other but the cutter heads are rotated synchronously with each other while the respective cutter heads are driven independently of each other, by electrically controlling the cutter drive motors.
In the control system (1), however, when one of the rotating cutter heads is suddenly stopped when an abnormally large load is applied, cutter drive torque will act on one tooth of the large gear. For such a sufficient strength of each tooth for withstanding the load, the tooth dimensions such as width, thickness and the like must be sufficiently large, which will make it difficult in practice to manufacture such gears.
In the control system (2), all the load torques to the rotating cutter heads are not identical to each other but the load to the cutter heads varies due to differences in soil during excavation, so that a deviation in angle (difference in angle of rotation) is likely to occur between the adjoining rotating cutter heads due to a slight difference in speed of rotation between the cutter heads. This control system needs a correction against such angle of deviation. Furthermore, in case the rotating cutter heads are driven independently of each other by electric drive systems, respectively, each comprising plural motors with a reduction gear in order to disperse the drive torque, it is difficult to have the plural motors driving the rotating cutter heads share uniformly the torque even if the cutter head drive systems are controlled for synchronous rotation of the cutter heads. If the excavating resistances and rotating resistances to the cutter heads are out of balance between the cutter heads, it is not possible to limit the angle of deviation between the cutter heads to within the allowable range, which leads to an interference between the cutter heads.